Some existing electrophotographic color image forming apparatuses may include photosensitive drums and developing devices in the same number as colors of toner. Each of the photosensitive drums is configured to carry an electrostatic latent image thereon. Each of the developing devices includes a developing roller configured to rotate in contact with (either directly or indirectly with toner therebetween) a corresponding one of the photosensitive drums and supply toner to the electrostatic latent image thereon. The image forming apparatus may further include a transfer belt disposed facing the photosensitive drums. Toner images formed on the respective photosensitive drums are sequentially transferred onto a sheet of paper fed on the transfer belt.
Existing color image forming apparatuses may be configured such that all photosensitive drums rotate in contact with the transfer belt not only in color printing but also in monochrome printing. In this configuration, when the developing rollers rotate in accordance with rotation of the photosensitive drums, toner particles may rub against each other, resulting in toner deterioration. To prevent toner deterioration, an image forming apparatus may stop the developing rollers, except for the developing roller for black, in monochrome printing. However, when all photosensitive drums are still rotating in monochrome printing, toner between the photosensitive drums and the developing rollers, except for black, deteriorates and vanishes, and a strong friction is produced therebetween. As a result, in colors except for black, the developing rollers may be shaved locally (e.g., a surface or portion of the developing rollers may deteriorate) due to the continued rotation of photosensitive drums while the developing rollers are stopped.
To prevent the developing rollers from being shaved locally, a known image forming apparatus includes a separation mechanism configured to separate the photosensitive drums and the developing rollers for colors except for black. The separation mechanism includes separation members and gears to move the developing devices. In color printing, the separation mechanism is configured to bring all developing rollers in contact with the respective photosensitive drums (either directly or indirectly with toner therebetween). In monochrome printing, the separation mechanism is configured to bring only the developing roller for black in contact with the corresponding photosensitive drum and to separate the developing rollers for the other colors from the corresponding photosensitive drums.
Through the provision of the separation mechanism, the photosensitive drums and the respective driving rollers except for black do not rub against each other in monochrome printing. Thus, the wearing away of the developing rollers and toner deterioration can be reduced.
However, in the above described image forming apparatus, the separation mechanism may involve inclusion of many components and thus may be complicated in structure and/or costly.